The subclass Prototheria contains the egg-laying mammals, which are the most ancestral forms in the class Mammalia. There are only three extant species grouped into two families and a single order, the Monotremata. Despite bearing fewer species than most mammalian genera, the prototherians are so unique among mammals that there is little question that they represent a distinct and ancient branch of the mammmalian family tree. However, it is not clear how monotremes are related to the two other major lineages of mammals, marsupials (Metatheria) and placentals (Eutheria). Some evidence supports the hypothesis that prototherians form a clade with the marsupials, while other evidence suggests that prototherians are sister to a clade containing both marsupials and placentals. (Heckner, 1990; Janke et al., 1996; Janke, Xu, and Arnason, 1997; Killian et al., 2001; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)

 

Prototherians probably split from the lineage leading to other mammals sometime in the Mesozoic. They retain many characters of their therapsid ancestors (for example, a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented with humerus and femur held lateral to body, and a cloaca). The skulls of monotremes are almost birdlike in appearance, with a long rostrum and smooth external appearance. Modern monotremes lack teeth as adults; sutures are hard to see; the rostrum is elongate, beak-like, and covered by a leathery sheath; and lacrimal bones are absent. Monotremes have several important mammalian characters, however, including fur (but they lack vibrissae), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to lactate. (Heckner, 1990; Nowak, 1991; Vaughan, Ryan, and Czaplewski, 2000)